This invention relates to a method for inserting an elastomeric yarn and a yarn processing system.
A longitudinally elastomeric yarn may be processed in a knitting machine when the yarn is released from the yarn supply by a positive feeding device and is fed by the positive feeding device to the respective knitting system. In weaving machines and by weaving machine feeding devices until now only so-called covered elastomeric yarns of restricted longitudinal elasticity could be processed. Covered elastomeric yarns, however, show a stretching property with a stretching curve containing a xe2x80x9ckneexe2x80x9d, meaning that the covering of the yarn physically increases the stretch force from the knee onwards. During each insertion process in the weaving machine the yarn first is stretched until the knee is reached, and then the yarn can be inserted at relatively stable relationships. Uncovered or bare elastomeric yarns, e.g. natural rubber yarns or elastomeric yarns, however, cannot be processed by weaving machine weft yarn feeding devices for feeding a weaving machine due to the extreme yarn stretchability. The preparation of covered elastomeric yarns by specific spinning processes, however, is costly.
It is an object of the invention to provide a method for inserting an elastomeric yarn into a weaving machine as well as a yarn processing system apt to carry out the method, both allowing the processing of longitudinally elastic elastomeric yarn material as the weft yarn of a weaving machine in a reliable way and independent from the yarn stretchability.
According to the method of the invention, the elastomeric yarn is supplied from the yarn supply to the shuttleless weaving machine such that the yarn first is released from the yarn supply by a positive yarn feeding process, then is intermediately stored in adjacent windings at a winding speed which is synchronised to the positive feeding speed and in a predetermined relation to the positive feeding speed and in a stretched condition, and finally is released for each insertion cycle with a predetermined insertion length. As by the synchronisation between the positive feeding speed and the winding speed for the intermediate storing process of the windings, a predetermined stretched yarn condition will be adjusted, which condition is independent from the stretchability of the yarn in the windings, and since the stretch will be controlled for each insertion cycle by measurement of the yarn release length, the yarn now can be processed by the weaving machine independent from the stretchability of the yarn. In this fashion, bare elastomeric yarn material can be processed for the first time in shuttleless weaving machines. Bare elastomeric yarns, however, can be produced for fair costs and offer an expanded degree of freedom of fabric elasticity in the weft yarn direction. The expanded degree of freedom is advantageous for stretched denim fabric (stretch jeans). An extremely stretchable bare elastomeric yarn does not need to be inserted by each pick into the fabric. A bare elastomeric yarn even may be inserted together with a normal yarn in one and the same pick, e.g. for producing a plated fabric.
The positive feeding device and the yarn intermediate storing and measuring feeding device are co-operating in the yarn processing system with each other such that a substantially uniform stretched yarn condition is maintained in the windings which are intermediately stored for intermittent consumption by the weaving machine. The stretched yarn condition may be precisely controlled by matching the speeds of the positive yarn feeding process and the winding on process. Furthermore, a desired stretch can be adjusted also in the weft yarn, because the weaving machine is inserting the yarn only in the form of precisely measured longitudinal sections and by one longitudinal section per pick. The stretch in the remaining intermittently stored windings cannot relax in the direction of the weaving machine. Only during the insertion cycle, the windings are not under form-fit control in the direction into the weaving machine. However, then the withdrawal force and the friction during withdrawal will cause an active hindrance which substantially suppresses a yarn relaxation acting rearwardly into the windings.
It is advantageous for the method to adjust the insertion length shorter than the weaving width of the weaving machine. This allows the adjustment of a predetermined stretched condition in the weft yarn in the fabric, e.g. about 300%. So to speak, the stretched condition of the weft yarn is frozen in the fabric by the closing action of the shed after the beat-up action.
Expediently, the magnitude of the yarn stretch in the intermediately stored windings can be maintained substantially constant by the measurement of the insertion length during the insertions or picks and also the resting periods between subsequent picks. This means that substantially no yarn length will be pulled back from the weaving machine while yarn fed by the positive feeding device is wound on. This is assured either by the stopping device carrying out the longitudinal measurement or by the withdrawal force during a simultaneously occurring pick. During an insertion only the predetermined and already stretched yarn length is released with which the weaving machine generates a predetermined stretch effect in the fabric. Even extremely stretchable yarn material can be correctly controlled by matching the proportions between the positive feeding speed and the winding on speed and by a phase equalisation (synchronisation). This means that the positive feeding device will start, stop, accelerate, and decelerate its motion as the winding on element does.
A positive feeding device is structurally simple and of reliable function, if it is operating with a driven conveying roller pressed against a rotatable yarn winding package or yarn bobbin. The drive of the conveying roller runs in synchronism with the winding drive and unreels the yarn. The positive feeding device can be mounted to the measuring feeding device and can be driven from the measuring feeding device, e.g. by a belt drive system or a gear transmission. The relation between the positive feeding speed and the winding on speed should be adjustable.
In the yarn intermediate storing and measuring feeding device the already stretched yarn is wound onto a stationary storage body in adjacent windings expediently with yarn separation, i.e., with axial intermediate distances between the adjacent windings. The yarn then will be held under stretch between the winding on element and the stop element. During each pick only the predetermined yarn length is released. Then the withdrawal force and the friction at the storage body will act in a yarn supporting way until the stop element again will support the remaining yarn windings against a relaxation. The stretched condition in the weft yarn in the fabric is adjusted then by releasing only a length which is defined in proportion to the weaving width. Particularly for processing bare elastomeric yarn material in a gripper weaving machine, it is expedient to provide a controlled yarn brake which, e.g. in the yarn transition phase between the bringer gripper and the taker gripper temporarily produces a predetermined braking effect in order to support the transition operation.
A monitoring assembly expediently is provided between the positive feeding device and the measuring feeding device. The monitoring assembly senses the yarn in a contactless fashion and does not generate significant additional friction for the yarn. In case of a yarn breakage or of an emptied yarn winding package or yarn bobbin, a stop signal is generated for the weaving machine.